Recently we discovered that signaling from blood vessels to surrounding cells can modulate organogenesis. Specifically, we identified signals from aortal endothelial cells that are required for the proper differentiation of pancreatic islet beta cells. Growing evidence indicates that endothelial cells create a "vascular niche" that provides signals important to developing stem cells in the brain and in the bone marrow. It is becoming increasingly clear that signals from blood vessel cells are critical to tissue growth, morphogenesis and maintenance. In this proposal, we aim to identify molecules that relay cell-cell communication signals between blood vessels and pancreatic endoderm. In particular, we want to characterize the endothelial signal responsible for the induction of insulin. To accomplish this goal, we propose three specific aims: (1) To determine general molecular characteristics of the EC-pancreas signal, by examining whether it is secreted or cell-tethered, and whether it is a protein. Using an in vitro assay we developed as a tool, we will test conditioned media, assay whether the signal can cross filters with fixed pore sizes, and test whether the signal is a protein. This aim will give us broad characteristics to help identify the molecular identity of the signal. (2) Examine candidates from a differential microarray screen we have already carried out, which compares inductive to non-inductive endothelial cells to find common sequences that encode the EC inductive signal. In this aim, we will confirm and characterize existing candidate molecules. (3) Functionally assay candidate molecules using standard in vitro and in vivo gain- and loss-of-function experiments. In particular, we will investigate the role of BMP signaling during insulin induction, since preliminary data indicates that BMP overexpression can induce insulin ectopically. We suggest that in addition to elucidating one of the steps critical to the differentiation of the islets of Langerhans, the data obtained from this screen will be more broadly applicable towards understanding basic vascular biology, angiogenesis, and how blood vessels are likely to exert global signals to other organs and tissues. Ultimately, our goal is to elucidate fundamental mechanisms driving beta cell development, and so that these can be emulated in the laboratory for the generation of transplantable replacement islets for patients with Type I diabetes.